This invention relates to a liquid level measurement system, and in particular to such a system employing a fixed, generally vertical linear member in which signals are induced indicative of the level of the liquid.
Liquid level measuring systems are known which include a vertically mounted linear member extending into the liquid and a movable member in the form of a float slidably mounted on the linear member. Such systems have almost all been analog systems, generally of the variable resistance type. Some of the better systems have used reed switches, activated by a magnet in the float, as shunts to provide a stepped output, as for example in Levins U.S. Pat. No. 3,200,645 (1965). These systems suffer the drawbacks common to analog systems, and the read switch systems have the further problems and expense of accurately positioning a reed switch for each measurable position of the float's travel.
One absolute linear position measurement system used in a liquid level measuring device is disclosed in French Pat. No. 1,419,367 (1965), in which separate floats and switches are provided at each discrete position along the linear member and all of the switches are connected in parallel so that each position is indicated by the closing of an individual circuit. Another absolute system, described in Ordorica et al, U.S. Pat. No. 3,154,946 (1964), uses a separate transformer at each discrete position. The transformers are sequentially activated to determine which one is coupled through a magnetic float and therefore produces a higher amplitude output pulse. Such systems are obviously subject to severe practical limitations.
Adapting other known position measuring systems to liquid level measurement also appears impractical. Optical linear position measuring systems are not very well adapted for use in liquid level measuring devices. An optical transducer is not compatible with immersion in many liquids, nor is the coded linear scale, and both are sensitive to contamination, opacity, and inhomogeneities in the liquid. Increment counting systems, both of the optical type and of the type sold under the registered trademark Inductosyn, which utilizes inductive coupling between conductors in a slide and in a linear member, are likewise not well suited to liquid level measurement. The difficulties in raising (or lowering) the float to a reference position periodically or after a power outage severely limits the usefulness of any increment counting system. Finally, both optical systems and inductively coupled systems require electrical connections to a float. The weight and elasticity of the power line must be compensated for as the float rises and falls, and in an Inductosyn system inductive coupling of this line must be minimized.
The failure of the prior art to provide an acceptable linear position measurment system for liquid level measurement is attested by the number of indirect level measurement systems which have been proposed and actually used. One widely used system eliminates the linear member, and the position of a float is determined by the length of a cable attached to the float. The cable is played out over a pulley above the liquid, and the excess cable is wound and stored on a drum. The length of cable played out is determined by digital or analog sensing devices associated with the cable, the pulley or the drum. Such a system is shown, for example, in Wright, U.S. Pat. No. 3,069,656 (1962). This type of system has a number of moving parts, requires careful compensation for a number of variables, and is subject to inaccuracies caused by wear of the moving parts, stretching of the cable, and the like. Even less direct systems involve the use of ranging apparatus for reflecting signals off the surface of the liquid, the placing of capacitance probes at intervals along the side of the liquid-containing vessel, the weighing of the entire vessel by means of load cells under the vessel, or determining the vessel's contents by means of pressure transducers at the bottom of the vessel. All of these devices have limited range, have limited accuracy, require careful field installation or compensation for the particular liquid being measured, and are subject to long term drift.